Structure of superconductive wiring having SiAlON buffer layer thereunder

ABSTRACT

The structure of a wiring according to the present invention has a buffer layer interposed between an insulating layer and a wiring of a superconductive ceramic material, and the buffer layer hardly reacts on the superconductive ceramic material in a high temperature ambient, so that the superconductive ceramic material does not lose the superconductivity due to an influence of the buffer layer on the superconductive ceramic material during the formation stage of the wiring.

TECHNICAL FIELD

This invention relates to a structure of a superconductive wiring and,more particularly, to a structure of a superconductive wiring used forfabricating a thin film integrated circuit, a thick film integratedcircuit and so forth.

BACKGROUND ART

An example of the prior art superconductive wiring is shown in FIG. 1.The structure of the wiring is disclosed in the paper entitled as"Technology of Superconductive Wiring on Alumina Substrate", ElectronicParts and Material published by Association of Industrial Investigation,1987 August, pages 89 to 92.

Referring to FIG. 1, reference numeral 1 designates a high purityalumina substrate, and the high purity alumina substrate is sometimesabbreviated as FGA (Fine Grained Alumina) substrate. On the high purityalumina substrate 1 a thick film wiring 2 is formed. The process offorming the thick film wiring 2 starts with preparation of a powder ofyttria represented by the molecular formula of Y₂ O₃, a powder of thebarium oxide represented by the molecular formula of BaO and a powder ofthe copper oxide represented by the molecular formula of CuO. Thesepowders are calcined to produce a bulk solid of a superconductiveceramic material. The bulk solid of the superconductive ceramic materialis pulverized to produce a powder of the superconductive ceramicmaterial, and the powder of the superconductive ceramic material ismixed into an organic vehicle, so that a paste is formed. The paste isprinted on the high purity alumina substrate 1 by using a screenprinting technique, and, thereafter, the high purity alumina substrateis placed in the atmospheric ambient at about 800 degrees to about 1000degrees in centigrade for sintering, then a thick film wiring strip 2being formed.

However, a problem is encountered in the quality of the substrateusable. Namely, even if the paste of the superconductive ceramicmaterial is printed on a low purity alumina substrate available in themarket and a thick film wiring strip is, then, produced through aprocess sequence similar to that described above, the thick film wiringstrip does not show any superconductivity. This is because of the factthat the low purity alumina substrate available in the market contains alarge amount of impurities such as silicon dioxide. It is figured thatthe impurities react on the paste during the sintering stage and, forthis reason, an occurrence of the superconductivity is suppressed in thethick film wiring strip. Such a phenomenon is reported in JAPANESEJOURNAL OF APPLIED PHYSICS, 1987 May, VOL. 26 No. 5, Table 1 in pageL761.

It is therefore an object of the present invention to provide astructure of a superconductive wiring feasible for fabrication on a lowpurity substrate available in the market.

DISCLOSURE OF INVENTION

A structure of a superconductive wiring according to the presentinvention comprises an insulating layer, a buffer layer formed on theinsulating layer, and a wiring pattern having a wiring strip formed of asuperconductive ceramic material containing at least one elementselected from the group consisting of scandium, yttrium and lanthanides,at least one alkaline earth metal, copper and oxygen, and the bufferlayer is formed of a substance hardly reacting on the superconductiveceramic material in a high temperature ambient, in which the substanceis represented by the chemical formula SiAlON.

The superconductive ceramic material may contain a plurality of elementsselected from the group consisting of scandium, yttrium and lanthanides,a plurality of alkaline earth metals, copper and oxygen.

The aforementioned structure of the superconductive wiring according tothe present invention is provided with the buffer layer interposedbetween the insulating layer and the wiring pattern, and the bufferlayer hardly reacts on the superconductive ceramic material in a hightemperature ambient. For this reason, when the wiring strip of thesuperconductive ceramic material is formed on the buffer layer, thewiring strip continues to show the superconductivity even though theinsulating layer is low in purity. This results in that a thick filmintegrated circuit is capable of being formed on, for example, aninexpensive low purity substrate, and, accordingly, the thick filmintegrated circuit is decreased in the production cost.

If the substance used for the buffer layer is larger in thermalexpansion coefficient than the substance used for the insulating layerbut smaller than the superconductive ceramic material, the thermalstresses between the wiring pattern and the buffer layer and between thebuffer layer and the insulating layer are decreased, and, for thisreason, the structure of the superconductive wiring is prevented from apeeling and so forth. Moreover, if the buffer layer is formed of asubstance with a perovskite crystal structure, the superconductiveceramic material is grown on the buffer layer in a lattice matchedstructure, and, accordingly, the superconductive ceramic material ischemically stable. Additionally, the superconductive ceramic materialgrown on the buffer layer with the perovskite crystal structure issuperior in electric properties, and a high current density is by way ofexample achieved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross sectional view showing the structure of the prior artsuperconductive wiring, and

FIG. 2 is a cross sectional view showing the structure of asuperconductive wiring according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 2, there is shown the structure of a superconductivewiring according to the present invention. The structure shown in FIG. 2is a kind of wiring incorporated in a thick film integrated circuit andcomprises a low purity alumina substrate 11 available in the market andserving as an insulating layer, a buffer layer 12 formed on the aluminasubstrate 11, and a wiring pattern 13 on the buffer layer 12. However, apassivation film may cover the buffer layer 12 as well as the wiringpattern 13 in another example.

Although the alumina substrate 11 is used in the example shown in FIG.2, the insulating layer would be formed of silicon dioxide in the casewhere the present invention is applied to a thin film integratedcircuit. The alumina has a thermal expansion coefficient of about8.8×10⁻⁶ /degree in centigrade, and silicon dioxide is averaged in thethermal expansion coefficient to be about 12×10⁶ /degree in centigrade.

The wiring pattern 13 is formed of an oxide containing at least oneelement selected from the group consisting of scandium, yttrium andlanthanides, at least an alkaline earth metal such as, for example,calcium, strontium, barium or radium, and copper, and such an oxide isknown as a superconductive ceramic material. One of the superconductiveceramic materials is, by way of example, an yttrium-barium-copper oxide,and the thermal expansion coefficient thereof is about 17×10⁻⁶ /degreein centigrade within the range from about 400 degrees to about 800degrees in centigrade. Other examples of the superconductive ceramicmaterial are oxides respectively represented by the molecular formula of(LaBa)₂ CuO and the molecular formula of YbBa₂ Cu₃ O₆. (Gd₀.5 Ho₀.5)₁.0(Ba₀.9 Sr₀.1)₂.0 Cu₃.0 O₆.9 is an example of the oxide containing aplurality of elements selected from the group consisting of scandium,yttrium and lanthanides, a plurality of alkaline earth metals, andcopper. Additionally, it is unnecessary to the wiring pattern 13 in itsentirety to be formed of a superconductive ceramic material, but awiring strip of the superconductive ceramic material should be containedtherein. In this instance, only the wiring strip of the superconductiveceramic material may be formed on the buffer layer 12.

The buffer layer 12 is formed of a substance which does not react on thesuperconductive ceramic material in a high temperature ambient of, forexample, 800 degrees in centigrade. Some examples of such a substanceare oxides represented by the molecular formula of MgO and the molecularformula of SrTiO₃, respectively, and the yttrium stabilized zirconium(which is abbreviated as YSZ). These substances respectively havethermal expansion coefficients of about 14×10⁻⁶ /degree in centigrade,about 9×10⁻⁶ /degree in centigrade and about 10×10⁻⁶ /degree incentigrade. Other substances available for the buffer layer 12 areoxides represented by the molecular formulae of BaTiO₃, BaZrO₃, CaTiO₃,SrZrO₃, PbTiO₃, and the thermal expansion coefficients thereof are about14×10⁶ /degree in centigrade, about 8×10⁻⁶ /degree in centigrade,14×10⁻⁶ /degree in centigrade, about 9×10⁻⁶ /degree in centigrade, about25×10.sup.×6 /degree in centigrade and about 32×10⁻⁶ /degree incentigrade. The oxide represented by the molecular formula of SrTiO₃ hasa perovskite crystal structure and, for this reason, is desirable togrow the crystal structure of a superconductive ceramic material.Namely, the oxide represented by the molecular formula of SrTiO₃ issimilar in crystal structure to the superconductive ceramic materialwith a perovskite crystal structure of the oxide defect type, and,moreover, the lattice constant is nearly equal to that of thesuperconductive ceramic material, so that the superconductive ceramicmaterial is improved in orientation. Moreover, the material representedby the molecular formula of SiAlON is superior in corrosion resistance,and, accordingly, cracks are less liable to take place due to heatrepetition. In the case where the buffer layer of a magnesium oxide isformed on the alumina substrate or a silicon dioxide layer, since thethermal expansion coefficient of the magnesium oxide is greater than thethermal expansion coefficient but less than the thermal expansioncoefficient of the superconductive ceramic material, thermal stressesbetween the alumina substrate and the buffer layer and between thebuffer layer and the wiring pattern are decreased in value in comparisonwith the thermal stress produced in the prior art structure, and,accordingly, peeling and/or cracks are less liable to take place. Instill another example, the buffer layer may be formed of a conductivesubstance such as a noble metal of, for example, platinum.

The above mentioned buffer layer may be formed by using a screenprinting technique followed by a sintering stage or by using asputtering technique. Description is hereinunder made for a processsequence where the sputtering technique is applied to the formation ofthe buffer layer.

The process starts with preparation of a target of a superconductiveceramic material, a target of the yttria stabilized zirconium and analumina substrate 11. First, a solution of photoresist is dropped ontothe alumina substrate 11, then being spun at about 5000 rpm for coatingwith a photoresist film. The photoresist film is baked to form aphotoresist layer, and the photoresist layer is exposed through a mask,then partially removing a portion shielded with the mask in thedevelopment stage. As a result, the alumina substrate 11 is partlyexposed. Then, the target of the yttria stabilized zirconium issputtered, so that a yttria stabilized zirconia layer is deposited onthe photoresist layer as well as the exposed part of the aluminasubstrate 11. When the photoresist layer is removed by using a lift-offtechnique, a buffer layer 12 is patterned on the alumina substrate 11.

In a similar manner, a photoresist layer of about 10 microns thick isformed in such a manner as to cover the entire structure, and,thereafter, the photoresist layer is patterned by using lithographictechniques. The resultant structure is placed in an argon ambient of,for example, about 10⁻⁴ torr to 10⁻¹ torr, and the target of thesuperconductive ceramic material is sputtered therein. As a result, thesuperconductive ceramic layer is deposited thereon, and a wiring pattern13 of the superconductive ceramic material is formed on the buffer layer12 after the removal of the photoresist layer by using the lift-offtechnique.

INDUSTRIAL APPLICABILITY

The structure of the superconductive wiring according to the presentinvention serves as an interconnection for semiconductor chipsincorporated in a thick film integrated circuit as well as aninterconnection for component circuit elements incorporated in a thinfilm integrated circuit.

We claim:
 1. A superconductive wiring structure comprising:an insulatinglayer, a buffer layer formed on said insulating layer, and a wiringpattern formed on said buffer layer having a wiring strip formed of asuperconductive ceramic material containing at least one elementselected from the group consisting of scandium, yttrium and lanthanides,at least one alkaline earth metal, copper and oxygen, wherein saidbuffer layer is formed of SiAlON, which is a substance hardly reactingon said superconductive ceramic material in a high temperature ambient.2. A structure of a superconductive wiring as set forth in claim 1, saidhigh temperature ambient is about 800 degrees in centigrade.
 3. Astructure of a superconductive wiring as set forth in claim 1, in whichsaid insulating layer is formed of a substance selected from the groupconsisting of an alumina and silicon dioxide.
 4. A superconductivewiring structure comprising:an insulating layer, a buffer layer formedon said insulating layer, and a wiring pattern formed on said bufferlayer and having a wiring strip formed of a superconductive ceramicmaterial containing a plurality of elements selected from the groupconsisting of scandium, yttrium and lanthanides, a plurality of alkalineearth metals, copper and oxygen, wherein said buffer layer is formed ofSiAlON material.